1. Field of the Invention:
The present invention relates to an autofocus device, and more particularly to an autofocus motor controlling and driving device provided in an automatic focus-adjusting system of a video camera or the like and adapted to effect control of a focus adjusting DC motor for driving a focus adjusting lens.
2. Description of the Related Art:
Conventionally, in an automatic focus adjusting system, if a distance measurement determines that the camera is in the in-focus state, a stop signal is supplied to a motor controlling and driving device, and an autofocus motor is stopped by opening or short-circuiting both terminals of the motor, thereby stopping the focus adjusting lens at the focused position.
However, in the case of the type in which the motor is stopped by opening both terminals of the motor, so-called overshooting may occur in which, even if a stop signal is output, the motor fails to stop immediately due to the inertia of the motor, thereby stopping beyond a desired stopping position. As a countermeasure against this problem, a type of speed control is effected in which the motor speed is decelerated in stages immediately before the in-focus position. However, even if such speed control is effected, it is difficult to completely eliminate the effect of the inertia of the motor, and the stopping position varies.
In addition, in order to mitigate the effect of the aforementioned inertia, brakes are applied to a motor by short-circuiting both terminals of the motor, thereby stopping the motor. In this type, however, there has been a problem in that power consumption increases.
In addition, speed control of a motor is generally effected in an automatic focus adjusting system. In other words, control is generally effected in such a manner that in cases where the stopping position deviates substantially from a focusing point, the speed before reaching the focusing point is increased by moving the motor at high speed, and when the motor approaches the focusing point, the motor speed is decreased so as to stop it within a predetermined insensitive zone with high accuracy.
If an attempt is made to improve accuracy, it is necessary to set the speed in the vicinity of the focusing point as low as is practical. However, if the speed is too low, the motor may stop midway, failing to reach the focusing point.
Conventionally, it has been necessary to set the driving conditions of the motor so that the driving is completely guaranteed under all conditions, i.e., in the presence of variations in the motor, variations in the load torque, and variations in circuits, at both low and high temperatures, and so forth. Accordingly, under most prevailing conditions, with respect to the guarantee of driving, it has been inevitable to sacrifice stopping accuracy in order to provide a large margin.
In a conventional automatic focus adjusting system, in cases where the subject is located at infinity, the lens is forcedly driven to the infinity end. It is then impossible to measure the distance accurately even if the lens is moved to the infinity end, so it is necessary to keep the lens located at the infinity end as long as the subject continues to be located at infinity. Meanwhile, if the lens is once moved to the infinity end in case of infinity, it is subsequently unnecessary to supply driving power to the motor in the direction of the infinity end, and the driving of the motor is stopped upon detection of its arrival at the infinity end.
As for the arrangement for realizing this stopping operation, there is a method whereby a switch is provided which blinks upon detection of the arrival of the lens at the infinity end, and the driving of the motor is stopped by this switch when the arrival of the lens at the infinity end is detected. In addition, there is another method whereby, while the subject is at infinity, the lens is driven in the direction of the infinity end for a fixed duration, and the lens is stopped after the fixed duration.
In different autofocus devices respectively employing the above-described two different methods, their control programs differ in accordance with the stopping methods adopted, in order to control the operation in accordance with the appropriate methods.